1. Field of the Invention
The present invention relates to a motor having a detector or sensor for detecting a rotational position of a rotor, and more particularly, to a motor having a shield structure for preventing magnetic flux leakage from entering the sensor.
2. Description of Related Art
Brushless motors having permanent magnet rotors are used for driving electric automobiles and hybrid vehicles. In this type of brushless motor, a detector or sensor detects the rotational position of the rotor to control the phases of currents applied to stator windings. When a magnetic sensor is used as the position sensor for detecting the rotational position of the rotor, magnetic flux leakage from the motor can adversely affect the detection precision causing the position sensor to be operated improperly and interfering with optimum operation of the motor. Further, since this magnetic flux leakage increases as the current to the motor winding is increased, the adverse influence of the magnetic flux leakage increases as the current increases.
To avoid or reduce the adverse influence of the magnetic flux leakage, the position sensor has sometimes been positioned in a region where the position sensor is not affected by the magnetic flux leakage. However, this generally requires that entire motor including the position sensor be increased in size in order to place the position sensor away from areas where magnetic flux is produced.
Alternatively it is known to avoid or reduce the adverse influence of the magnetic flux leakage by disposing a magnetic shield plate between the motor and the position sensor such as disclosed in Japanese Patent Application Laid-open No. HEI 11-78558. The magnetic shield plate is directly fixed to the motor housing by connecting the magnetic shield plate to the stator core through a mounting member. The prior art magnetic shield plate installed between the motor and the position sensor is still deficient in preventing increases in stray magnetic flux from causing flux changes in the position sensor when the stator current is increased. Therefore, in order to prevent stator flux changes from affecting the position sensor, it is necessary to increase the distance between the motor and the magnetic shield plate or between the magnetic shield plate and the position sensor. Thus, the entire motor including the position sensor is increased in size also in the case of employment of a magnetic shield.
The present invention has been achieved to solve these problems, and it is an object of the invention to provide a small motor in which magnetic flux leakage from a stator winding does not adversely affect a position sensor (magnetic sensor) which detects rotational position of a rotor.
It has been discovered that when the mounting member for a shield plate is a member having low relative permeability such as aluminum as in the prior art, a magnetic gap between the stator core and the shield plate results in magnetic leakage flux from the stator winding flowing from the magnetic shield plate toward other members producing the likelihood of magnetic leakage flux adversely affecting the position sensor.
According to one aspect of the invention, a motor includes a cylindrical rotor, a stator arranged at a predetermined distance from an outer periphery of the rotor and having a stator core and a plurality of stator windings arranged along a circumferential direction of the stator core at substantially equal distances from one another, a position sensor for detecting a rotational position of the rotor, and a shield member directly mounted against the stator core for shielding magnetic flux from the stator windings to the position sensor. Leakage magnetic flux generated by the stator windings passes in a closed loop including the shield member and the stator core thereby preventing the leakage magnetic flux in such closed loop from flowing from a gap between the shield member and the stator core to another member affecting the position sensor. Therefore, an error or detection failure of the position sensor due to the magnetic flux leakage is prevented.
According to a preferred form, the shield member is disposed so as to extend around the stator windings on all sides facing position sensing and indicating members. Therefore, it is possible to reliably contain leakage magnetic flux from the stator windings to closed loops to reduce the effect of stator winding magnetic flux leakage on position sensing.
According to another preferred form, the position sensor is a magnetic sensor, and the position sensor is disposed close to the outer periphery of the stator windings in a radial direction thereof. Therefore, an axial size of the motor with the position sensor can be shortened to make the motor more compact and the position sensor is less prone to be affected by the magnetic flux leakage from the stator winding.
According to still another preferred form, the position sensor detects the rotational position of the rotor through a position indicating member extending integrally from the rotor toward its outer periphery in the radial direction thereof By disposing the position indicating member close to the position sensor, the rotational position of the rotor can be accurately detected.
According to further preferred form, the position indicating member extends toward a radially outer peripheral side from the rotor on a side of the shield member axially opposite to the stator windings. Therefore, magnetic leakage flux coming from the stator windings is prevented by the shield plate from flowing to the position indicating member.
According to a still further preferred form, the position indicating member is a plate which at its periphery is bent axially over an outer peripheral side of the shield member and the stator windings and which has a plurality of detection teeth formed on the bent portion along a circumferential direction thereof opposing the position sensor detecting detection teeth to detect the rotational position of the rotor. Therefore, the stator winding leakage magnetic flux is prevented from flowing through the position indicating member disposed in the vicinity of the stator windings, and the motor can be made compact.
According to another preferred form, the stator core and the shield member or plate are integrally fastened to a motor housing which accommodates the rotor and the stator. Therefore, it is unnecessary to fix the stator core and the shield plate using another member, whereby the number of parts is reduced, and the motor can be made compact.
According to still another preferred form, the shield plate extends from the stator core along a peripheral side of the stator windings on a radially outer side of the motor in an axial direction away from the stator core, and extends radially inward over the stator windings up to a radially inner side of the stator windings. Accordingly, the magnetic flux leakage from the stator winding can reliably be blocked, since the shield member covers an outer peripheral side of the stator windings and a lateral side of the stator windings axially opposite the stator.
According to yet another preferred form, the portion of the shield plate located on the radially inner side of the stator windings extends axially toward the stator core. Therefore, a gap between the stator core and a radially inner edge of the shield plate is shortened, and magnetic flux leakage from the stator windings can be prevented from leaking to another member.
According to another preferred embodiment, the shield member extends from the stator core axially over an outer peripheral side of the stator windings, and then extends radially inward over a lateral side of the stator windings and over a lateral side portion of the rotor. Therefore, it is possible to shield the position sensing elements from both the magnetic flux leakage from the stator windings and the magnetic flux leakage from the rotor.
According to another preferred form, the shield member is formed from a ferromagnetic material. Therefore, it is possible to prevent the magnetic flux leakage from flowing to another member.
According to another preferred form, a distance between the stator core and the shield member is set shorter than a distance between the stator core and a position indicating member or plate. Therefore, it is possible to reliably flow magnetic flux coming from the stator core toward the shield member, to form the closed loop of the magnetic flux, and to prevent the magnetic flux from flowing to the position indicating member.
According to another preferred form, a non-magnetic heat conductive member is filled between the stator core, the stator windings and the shield member. The shield effect of the magnetic flux leakage can be maintained, and the heat of the stator winding can be transmitted to the stator core, the housing, and the like through the heat conductive member to enhance the heat radiation ability.
According to still another preferred form, the motor is applied to a hybrid vehicle driving apparatus. Accordingly, it is possible to obtain a compact hybrid vehicle having simple structure and high performance.
According to the present invention, the shield member mounted against the stator core is disposed between the stator windings and the position sensor such that any magnetic circuit followed by magnetic flux leakage from the stator windings on a side of the position sensor passes from the shield member to the stator core without any substantial gap. Therefore, the magnetic flux leakage from the stator windings is prevented from flowing to the position sensor and thus, the rotational position of the rotor can be detected accurately. In this case, since the shield member is directly mounted to the stator core, a magnetic circuit starting from the stator windings to the stator core through the shield member and back to the stator windings is formed, and the magnetic flux leakage from the stator windings is prevented from flowing to another member. Consequently, the shielding effect by the shield member against the stator winding magnetic flux is increased, and it is possible to increase the current in the stator windings.
Further, since an excellent shielding effect by the shield member can be expected, the position sensor can be disposed in the vicinity of the stator, and the distance between the stator winding and the shield member, or between the shield member and the position sensor can be shortened. That is, even if heavy current is applied to the stator windings, the position sensor can be disposed near the stator and thus, a compact motor can be obtained. Therefore, the shielding effect is enhanced as compared with a case in which a shield member not magnetically connected to the stator core is mounted.